A technique of flow cytometry has been used to analyze individual cells in which a cell suspension liquid is allowed to flow fast and irradiated with laser light and measurement is performed. A flow cytometer is known to be able to measure the relative size, the shape, and a difference in the internal structure of individual cells, and the fluorescent intensity and the type of fluorescence by performing fluorescent labeling, and thereby to be able to analyze the existence of cells and/or a cell cluster ratio of various cells in a cell cluster in a short time.
It is set in a manner such that a cell suspension liquid is made to flow fast along the center of a sheath liquid (sheath flow) and the flow is narrowed so that the cell suspension liquid passes through a conical flow passage, which results in that individual cells flow in line. If the cells that are flowing in line are irradiated with laser light, scattering light or fluorescence are emitted from the cells that are passing. An exposure time during which the cell is irradiated with laser light is merely several microseconds. Accordingly, damage to the cell is not problematic at all. The scattering light from the cell is detected as a forward scatter (FSC) in the same direction as a laser beam and a side-scattering light (SSC) at an angle of 90° to the laser beam. The FSC and SSC vary in light intensity, reflecting the size of a cell and the complexity of the internal structure of a cell, respectively. Fluorescence is the light radiating in a direction of an angle of 90° like the side-scattering light (SSC) and is extracted after splitting with the use of an optical filter. So, respective wavelength components thereof are detected.
For example, laser light is used as excitation light and irradiated onto a cell to detect a fluorescent pigment. A fluorescent pigment absorbs a certain wavelength and converts high energy light (short wavelength) to lower energy light (long wavelength). Each fluorescent pigment has the unique excitation wavelength distribution and the emission wavelength distribution. That is, it absorbs wavelengths of light in a certain range but emits radiating light (fluorescent light) with wavelengths in a certain range. The intensity of the obtained fluorescent light (fluorescent intensity) is measured using a detector, and the obtained value is converted into a digitized form and represented in a site gram or a histogram, together with the fluorescent intensities of other cells. For example, SITEGRAM shows information of scattering light and fluorescence that can be obtained from a cell, on the two-dimensional coordinate. In the histogram, for example, the abscissa axis indicates the intensity of light and the vertical axis indicates the number of cells.
As described above, as illustrated in FIG. 8, the conventional Flow Cytometer (FCM) acquires and analyzes measurement values of scattering light to recognize the size of cells and the difference in the internal structure of cells. As the scattering light, both forward scatter and side-scattering light are respectively acquired.
A cell cycle is the course that a daughter cell which has been produced by cell division becomes a mother cell and produces new daughter cells by its own cell division. The cell cycle can be divided into G1 phase, S phase, G2 phase, and M phase. Chromosome increases in number during the G2 and M phases in particular, so that the G2 and M phases are featured in that a cell nucleus or a cell grows bigger. Some specific cells such as stem cells are known that the size ratio of a cell nucleus to a cell is higher than those of general cells.    Patent Document 1: Japanese Patent No. 2973387 (Japanese Patent Application Laid-Open No. 6-323987)    Non-patent Literature 1: Cell Technology additional volume, Experimental Protocol Series, “New Edition Flow cytometry Free—from a multi-color analysis to a clone sorting—,” p. 113, (published by Shujunsha Co., Ltd, supervised by Hiromitsu Nakauchi).